How would u feel about ur bf or gf hiding things from u?

okk so ive been trying to help my friend out but im stuck. she said she asked her bf if hes ever hidden anything from her and he said yes and she asked when was the last time and she told me he said about 2 weeks ago but it was nothing serious. i tried to help her but i dont know what to tell her. she says her boyfriend tells her everything, but then again, hes hiding things from her... i did however tell her to ask him that if wasnt anything serious why did he hide it from her in the first place? please help thanks (: